Preheat the oven to 325 degrees F. Line a 9-by-13-inch baking dish with foil, leaving an overhang on 2 sides. Butter the foil.
Sift the flour, cocoa powder and salt into a medium bowl; set aside.
Melt the butter and chopped chocolate in a large saucepan over medium heat, stirring, until smooth, about 4 minutes. Remove from the heat; let cool slightly, then stir in the sugar. Mix in the beaten eggs until combined. Stir in the flour mixture and 1 cup chocolate chunks until just combined.
Pour the batter into the prepared dish and spread in an even layer. Sprinkle with the remaining 1/2 cup chocolate chunks. Bake the brownies until a toothpick inserted into the center comes out slightly moist, about 40 minutes. Transfer to a rack and let cool completely. Lift out of the pan, peel off the foil and cut into pieces.